The present invention relates generally to a leak standard assembly and, more particularly, to a portable leak standard capable of delivering high-purity fluid at a precise leak rate over an extended period of time.
Leak standards have been used for calibration of precision instruments such as leak detectors, mass spectrometers, off-gassing monitors, vacuum gauges, and various flow-type gauges. For example, in a leak detector in which helium is used in conjunction with a mass spectrometer for detecting leaks in a vacuum system, the mass spectrometer is adjusted to collect only helium ions to detect leaks in the vacuum system as small as 1.times.10.sup.-9 atm-cm.sup.3 /sec. To calibrate the detector for measuring such leakage, a leak standard capable of emitting a precise quantity of helium is utilized.
The leak standards heretofore used for the calibration of various instruments such as mentioned above primarily utilized a leak standard in which a glass capillary provided the leak valve for metering the specified quantity of gas. In the fabrication of such leak standards a section of a glass tube is heated and then drawn to provide a minute orifice through the drawn section. While leak standards utilizing such capillary tubes have been fairly successful, there are several shortcomings which detract from their overall desirability. It has been found to require the exercise of considerable skill to provide such capillary leaks from glass tubes due to the problems attendant with drawing the glass to form an orifice of a specified size. Also, the glass leak valves are susceptible to breakage during routine handling of the leak standard assembly. Other shortcomings with the leak standards utilizing glass capillaries are due to the incapabilities of utilizing relatively high-pressure, long-life reservoirs and of operation in environments at elevated temperatures. Further, with attendant temperature restrictions the reservoirs and other components of assembled leak standards employing glass capillaries cannot be properly cleansed of volatile impurities, such as water and nitrogen. In a conventional glass capillary leak standard a total impurity content of at least about 50,000 ppm is present so as to considerably detract from the efficiency of the standard when calibrating various instruments. The useful lifetime of the conventional leak standards is for about a period of two months since the safe reservoir capacity of these assemblies is at a pressure of only about two atmospheres absolute. Due to this relatively small reservoir capacity, the leak standards need to be refilled and recalibrated about every two months which results in a considerable expense in terms of man hours and dollars.